Liquid crystals are substances in liquid state and they show optical anisotropy like a crystal. Optical properties of the liquid crystals vary with a change of molecular arrangement by exterior stimulations such as an electric field, magnetic field, temperature,pressure,and the like. Various display elements, for example, electronic calculators, sensors, computers, televisions, transmission-type optical valves, etc. have been developed by using such properties of the liquid crystals.
Liquid crystal display elements can be fabricated to be more compact, provide larger screens and lower electric power consumption compared with the conventional Braun tube mode display elements. Accordingly, it is expected that liquid crystal display elements will become the main display elements in the relevant technical fields in the near future.
Conventional liquid crystal display elements include the twisted nematic (TN) display element and the supertwisted nematic (STN) display element, and the like. However, since such nematic liquid crystal display elements have low response speed and difficulties in demonstrating the gray scale, they are not used in forming large screens and in applications requiring a dynamic picture display. Therefore, smectic liquid crystals have been used in the display elements.
Among the smectic liquid crystals both the smectic A phase and the smectic C phase have a low degree of freedom. The liquid crystal of chiral smectic C phase has an asymmetric carbon atom which generates polarization when dipole moment is directed to one direction.
The resulting polarization is called spontaneous polarization. The liquid crystal with high spontaneous polarization is called a ferroelectric liquid crystal, which can be rapidly reversed by the polar change of exterior electromagnetic fields, and thus, has a high response time to present a dynamic picture, and also has high contrast and wide viewing angle. Therefore, it can be utilized as a direct view-type display element.
However, since the ferroelectric liquid crystal has poor chemical stability against light, moisture and heat, it is not well used in commercial applications.
In order to solve such defects, biphenyl ester-based or pyrimidine-based liquid crystals have been developed. However, they cannot satisfy the desired levels. Liquid crystal display elements can be prepared by dispersing liquid crystalline compounds in polymeric materials and changing the temperature or solubility of the resulting dispersion, curing the dispersion to form a film, and then sandwiching the film between glass plates coated with indium-tin oxides (ITO).
In the display elements, since the refractive index of the polymeric materials is different from that of the liquid crystals in the form of droplets, when light passes through the display elements, the incident light is scattered, so that the display elements are opaque.
However, when electric voltages are applied to the ITO glass plates, the liquid crystal droplets are aligned with the polymeric materials in the same direction, and thus, the light passes through. Accordingly, an image display is possible due to the change of light by the liquid crystals.
However, the conventional nematic liquid crystal display elements have various problems when used as a direct view-type.
That is, when electric voltages are applied, since the arrangement time from horizontal direction to vertical direction against electrodes requires more than several hundred msec, the response time is slow, and thus, there is difficulty in showing a dynamic picture. Also, when the liquid crystals are aligned, since the viewing angle is narrow, there is difficulty in watching the display screen from an off center direction.
Since the display elements using ferroelectric liquid crystals have high dielectric anisotropy and spontaneous polarization, due to the SmC phase having an asymmetric carbon atom in smectic liquid crystals, when electric voltages are applied thereto, the same have a rapid response time of not more than several hundred msec and a wide viewing angle. However, spontaneous polarization, viewing angle and response time rapidly decrease, or liquid crystalline properties become unstable, due to light, moisture or an increase in temperature. Furthermore, it is difficult to realize gray scale for coloration, to obtain homogeneous alignment, to control cell gap, and to obtain resistance against outside impact.